PHOSPHORYLATION OF HEPATIC RIBOSOMAL PROTEIN-S6 ON 80 AND 40-S-RIBOSOMES - PRIMARY STRUCTURE OF S6 IN THE REGION OF THE MAJOR PHOSPHORYLATION SITES FOR CAMP-DEPENDENT PROTEIN-KINASES

  • 1 January 1984
    • journal article
    • research article
    • Vol. 259  (4) , 2084-2091
Abstract
Rat liver ribosomes and 40S ribosomal subunits were phosphorylated with the purified catalytic subunit of cAMP-dependent protein kinase. Phosphorylation of ribosomal protein S6 plateaued at around 2 mol of phosphate/mol of protein with both substrates. Peptide map analyses showed that the most prominent phosphorylation sites associated with 40S substrates were the adjacent serines in the Arg-Leu-Ser-Ser-Leu-Arg segment of S6. The first serine residue appeared to be the preferred site as was established previously for 80 S ribosomes. Additional phosphorylation sites were apparent from the peptide maps. One of these was associated with the triphosphopeptide (termed T1a) having the sequence Arg-Leu-Ser-Ser-Leu-Arg-Ala-Ser-Thr-Ser-Lys. A larger fragment of S6 (termed T1c) isolated from mild tryptic digests of whole ribosomes, consisted of the T1a sequence extended by the sequence Ser-Glu-Glu-Ser-Gln-(Lys) at the COOH terminus. A comparison of the size and chromatographic and isoelectric focusing properties of the T1a/T1c peptides and prominent tryptic peptides of S6 from insulin-stimulated hepatocytes indicated a relationship between these peptides. Some of the potential phosphorylation sites in the T1a/T1c region of S6 may be phosphorylated by an insulin-regulated kinase in hepatocytes.